In the operation of a pulverized coal-fired boiler a significant fraction of the ash contained in the coal is deposited on the water walls of the combustion chamber and on the tubes of the convection section of the boiler. The ash deposits have a low thermal conductivity, modify the radiative properties of the surfaces and insulate the tubes from the flame. Both of these effects interfere with the efficient flame and gas-to-tube heat transfer.
Uncontrolled accumulation of ash often assumes catastrophic proportions necessitating boiler shutdowns. Often physical damage results to tubes in the furnace hopper when large masses of ash detach and fall there.
As a result of the generally decreased heat fluxes a larger heat transfer surface is required than otherwise would be the case. The increased use of fouling-type coals has led to a substantial increase in furnace size for a particular load, leading to an increased initial capital cost.
Boiler operation is controlled by maintaining superheat steam temperature and flow rate by use of the available operating variables, such as, gas recirculation, burner tilt, gas tempering and steam atemperation. A need for frequent use of these adjustments is indicative of uneconomic operation.
The build up of ash on the furnace walls is controlled by the intermittent operation of soot blowers, which remove the built-up ash from the walls. At the present time the boiler operator is not provided with any direct measurement of the degree of fouling of the combustion chamber and convection section.
The degree of fouling, in general, however, is random and unpredictable with respect to its distribution on the various parts of the furnace walls, and also as to its severity at any one point. In the mode of operation, as practised according to the presently available state of the art, soot blower actuation is based on operator judgment of the indirect evidence from superheated steam and economizer temperature, burner position, and/or amount of gas recirculation and steam atemperation.
Because the boiler response time (i.e., the time where changes in degree of fouling are reflected in these variables) is long, control is erratic. Moreover, in order to avoid catastrophic loss of control, boilers are designed with larger furnaces than they otherwise might need to be. If methods and instrumentation were provided to directly monitor the degree and distribution of fouling, both the boiler control would be improved, and smaller and therefore less costly furnaces would prove adequate. As far as the applicants are aware, there has been no development to date of ash deposit-measuring instrumentation to provide such means.
Similar problems arise with other combustion operations, such as, in kraft mill recovery boilers and waste heat boilers in metallurgical applications, wherein ash is deposited in the combustion chamber and/or on heat transfer ductwork.